1. Field of the Invention
The invention relates generally to photogrammetric analysis. More specifically, the invention relates to targets for non-intrusive photogrammetric analysis of surfaces and structures.
2. Background Art
Photogrammetry is a three dimensional coordinate measuring technique that uses photographs as the fundamental medium for measurement. Over recent years, the technique has emerged as a popular measurement tool for high precision measurement. It has a wide variety of applications in industries such as aerospace, automotive, shipbuilding, nuclear, communications, etc. Triangulation is the fundamental principle used by photogrammetry. “Lines of Sight” are developed by taking photographs of the object of interest from at least two different locations. The lines of sight are mathematically intersected to produce the three dimensional coordinates of the object. The coordinates are then analyzed for such things as defects in the shape of the object.
FIG. 1 shows an example of a prior art photogrammetric system that is analyzing a parabolic dish antenna. The principle of the photography portion 10 of the system is relatively straightforward. A light source 12 illuminates the object 14. The source typically uses a high intensity light that is generated in the form of a flash or a strobe light. In this example, the object 14 is a parabolic shaped dish antenna. The object 14 is covered with reflective targets. FIG. 2 shows a prior art example of a section 20 of the object 14 that is covered with reflective targets 24. The reflective targets 24 are adhesive dots that have a high contrast to the surface 22 of the object. As light from the light source 12 is reflected back from the targets 24 on the object 14, two cameras 16 record the reflection and the coordinates of the target are calculated by a computer (not shown).
Once the coordinates of the targets are determined, three dimensional depictions of the targets and the surface of the object are generated. FIGS. 3A-3C shown examples of computer generated three dimensional depictions of the shape of the object. FIG. 3A shows a depiction 30 of the coordinates of several rows of targets radiating outwards from the center of the object. Eventually, as shown in FIG. 3B, enough coordinates are recorded to generate a complete three dimensional depiction 32 of the object. Once this is complete, defects in the shape of the object 34 can be identified and highlighted by the computer. Ultimately, entire areas of deformation 42 of the object are identified and depicted 40 by the computer as shown in FIG. 3C.
For an antenna, photogrammetric analysis can be used to measure such features and characteristics as: the antenna surface deformation; the feed alignment; the shape analysis; and damage after shipping. However for thin film applications that are designed for use in space such as solar sails or inflatable antennas, photogrammetric analysis is not often accurate. These objects are made of very thin films of material that tend to deform when target reflectors are placed on their surface. The presence of these reflectors with their mass tends to distort their shape and thus lead to inaccurate measurements of the surface of such objects. This distortion due to the mass of the reflectors can influence the object both statically and dynamically. The current reflective targets can also interfere with the packaging and deployment of film structures. The mass of the current targets interfere with the mass and stiffness characteristics of the parent material affecting the measurement results of the structure. Consequently, a non-intrusive reflector is needed in order to properly analyze thin film objects with photogrammetry.